1. Field of the Invention
The present invention relates to a color cathode-ray tube apparatus and, more particularly, to a color cathode-ray tube apparatus which excels in convergence characteristic.
2. Description of the Prior Art
FIG. 1 shows an ordinary color cathode-ray tube which has been known hitherto. As is evident from this figure, the tube (i.e., main component) of the color cathode-ray apparatus comprises a panel 3 having a phosphor screen 2, a funnel 4 coupled to the panel 3, and a neck 5 extending from the funnel 4.
The neck 5 contains an electron gun assembly 6 for generating electron beams. A deflection yoke 7 is on the outer peripheries of the junction of the funnel 4 and the neck 5. The panel 3 and the funnel 4, connected to each other, form an envelope. Within the envelope, a frame 10 holds a shadow mask 9 having a number of apertures 8, such that the shadow mask 9 is spaced apart from the phosphor screen 2 by a predetermined distance. A magnetic shield 11 is secured to the frame 10. An inner electroconductive layer 12 is coated on the inner surface of the funnel 4 and also on the inner surface of the front portion of the neck 5. An outer electroconductive layer 13 is coated on the outer surface of the funnel 4. Anode terminals (not shown) are connected to the funnel 4. Outside the envelope, a drive-signal generating circuit 14 is located for applying an appropriate voltage to the electron gun assembly 6 and to an anode electrode, and for supplying drive signals to the deflection yoke 7.
The phosphor screen 2 consists of red-light emitting phosphor elements, green-light emitting phosphor elements, and blue-light emitting phosphor elements--all either stripes or dots, formed on the inner surface of the panel 3. The electron gun assembly 6 comprises a beam-shaping unit and an electron lens unit. The beam-shaping unit generates three parallel electron beams B.sub.R, B.sub.G and B.sub.B, adjusts the shapes of these beams, and accelerates them. The electron lens unit converges or focuses each of three beams. The deflection yoke 7 applies a magnetic field to the beams B.sub.R, B.sub.G and B.sub.B emitted from the electron gun assembly 6. The deflected electron beams pass through the shadow mask 9 and land on a red-light emitting phosphor element, a green-light emitting phosphor element, and a blue-light emitting phosphor element, respectively. Excited with the beams, the phosphor elements emit red light, green light, and blue light. In other words, the deflected electron beams B.sub.R, B.sub.G and B.sub.B scan the phosphor screen 2, whereby the screen 2 displays a color image.
The deflection yoke 7 has a horizontal deflection coil for generating a magnetic field which deflects electron beams in the horizontal direction, and a vertical deflection coil for generating a magnetic field which deflects electron beams in the vertical direction.
The phosphor screen 2 can be either an in-line stripe type shown in FIG. 2A or a delta dot type shown in FIG. 2B. As is shown in FIG. 2A, the in-line stripe screen has a number of parallel black strips 21 and groups of parallel phosphor strips coated among the black strips 21, each group consisting of a red-light emitting strip 22, a green-light emitting strip 23, and a blue-light emitting strip 24. As is shown in FIG. 2B, the delta dot screen has a black layer 21 having round openings, and groups of phosphor dots formed in the openings of the layer 21, each group made of a red-light emitting dot 22, a green-light emitting dot 23, and a blue-light emitting dot 24. Whichever type the screen 2 is, it cannot form a color image having high color purity or high brightness, unless three electron beams 25, 26, and 27 correctly land on the phosphor strips or dots 22, 23, and 24, respectively.
In recent years, more and more phosphor elements are formed in unit area to form high-quality color images, and more light each phosphor element is required to emit to form bright color images. Hence, the margin for erroneous beam landing cannot help but be limited. This margin limited, it is now necessary to minimize even the erroneous beam landing caused by mutual repulsion of electron beams, which is far less than the erroneous beam landing resulting from excessive or insufficient beam deflection.